that is all strange. once you set a loop range you certainly don't need to reset it over when you wish to engage in recording.
just try the follwing steps, which might seem overwhelming for a first timer but hopefully tend to get intuitive and obvious later on:
check that View/Options.../Transport/Loop recording mode (takes) is set to "First" or "Last", then set your loop range as usual, just once; you can also set a punch in/out range if you will, but that's not the point here now;
arm your MIDI track for recording (the [R] track button is lit red) and press main red button to prepare recording; if session is untitled new or blank it ask you the name that also serves as for file-name prefix;
connect your MIDI (piano keyboard?) controller output into Qtractor MIDI input port (you can do this whenever before, but here is your last chance);
make sure your MIDI controller sends on the same MIDI channel to the armed MIDI track, or you can check Track/Auto Monitor on and click or select the desired target MIDI track as current track (that will make all incoming MIDI filtered to the current track channel, automagically);
start playback so recording starts rolling... hit some keys, watch events being drawn under a red shaded strip while on the mixer strip the metters pump up and down as you're playing the piano keyboard keys; play as much as long for the play-head turn around a couple of cycles over the loop interval--each turn will be called and numbered as one take; hit stop;
now right-click anywhere on the resulting recorded clip and head for context-menu Clip/Take sub-menu, or you can just try pressing Shift+T to cycle between takes, starting from the one you chose on 1.
that is all strange. once you set a loop range you certainly don't need to reset it over when you wish to engage in recording.
just try the follwing steps, which might seem overwhelming for a first timer but hopefully tend to get intuitive and obvious later on:
or else you can always find some more help on the qtractor wiki, this section in particular and, who knows, give a hand or two :)
hth.
cheers